In this project we plan to evaluate the interaction between estrogen and the active vitamin D metabolite, 1,25(OH)2D, in the etiology of breast cancer, focussing on the role of insulin like growth factors (IGFs) as mediators of this interaction with respect to tumor cell proliferation. The hypothesis to be tested is that the proproliferative action of physiologic concentrations of 1,25(OH)2D on breast cancer development in the presence of estrogen is mediated by IGFs and/or their receptors and binding proteins whose production is controlled by 1,25(OH)2D and estrogen. The basis upon which this hypothesis was formulated is that 1,25(OH)2D stimulates proliferation of estrogen receptor positive (ER+) cell lines at low concentrations when estrogen is present, but is antiproliferative at these same concentrations in the presence of an antiestrogen or when added to estrogen receptor negative (ER-) breast cancer cells. Since IGFs, their receptors, and binding proteins are produced by breast cancer cells, regulate breast cancer cell growth, and vary in their response to estrogen in estrogen receptor positive and negative cell lines, IGFs and their receptors/binding proteins provide an attractive mechanism by which the estrogen/1,25(OH)2D regulation of breast cancer growth could be mediated. Three specific aims to test this hypothesis are proposed: determine the mRNA and protein levels of IGF-2, IGF-1 receptor (IGF-IR), and the IGF binding proteins (IGFBPs) in MCF-7 (ER+) and MDA-MB-453 (ER-) cell lines in the presence of 17beta-estradiol (E2) and/or the antiestrogen ICl 164384; determine the ability of 1,25(OH)2D at proproliferative (10[-11]M) and antiproliferative (10[-8]M) concentrations to regulate estrogen and/or antiestrogen induced changes in IGF-2, IGF-1R, and IGFBP production in the ER+ and ER- cell lines; and determine whether the interaction between 1,25(OH)2D and estrogen on the IGF system in vitro is predictive of their role in regulating breast cancer growth in vivo.